What happened in a half-filled Chinnaswamy Stadium on Wednesday was not just the finest day in the history of Irish cricket. It was, certainly, a result and a performance better than their mythic win over a well-entertained West Indies team on a damp Sion Mills wicket in County Tyrone 42 years ago, and more complete than their upset of Pakistan in the last World Cup. But it was also a seismic event that shredded English complacency bordering on arrogance, a cricket match to warm the French cockles of Marc Lièvremont.

It was a victory, too, that buzzed through all electronic conduits with the message that this World Cup – derided beforehand as loaded with soft and irrelevant fixtures – is no longer a place for comfortable assumptions.

England got ahead of themselves. There can be no other explanation for their capitulation from the heights of 327 against a team who only five days ago were bowled out for 178 in 45 overs when losing to Bangladesh, and who were following the script here until a man with pink hair and a burning bat magically rejuvenated them with the fastest century in the tournament's history. What Tamim Iqbal did to Ireland, Kevin O'Brien did to England, even more so.

Clearly, Ireland were assumed to be making up the numbers against a side who, just a couple of months ago, humbled Australia to retain the Ashes. That nobody informed the Irish of their status became obvious as they rudely imposed themselves on their betters.

As Nasser Hussain observed: "The only number you need to know about is two. Two points to Ireland, none to England."

All of a sudden, there is no reliable form line. While, in all probability, the grand order will not be disturbed so dramatically again, every match up to the quarter-final stage will carry a frisson of doubt. That can be no bad thing.

If the tournament was going to be a trial for the minnows, England now must share some of the angst. Since the Ashes, they have been invaded by a virus that left them deflated in the meaningless end-of-tour one-day series against Australia and broke out again against the Netherlands at the start of this World Cup.

Nobody in the aftermath on Wednesday seemed able to put a name to the malaise. It was not tiredness, we were assured. Nor was it a lack of diligence on the practice field, said those close to the action. However, through the prism of television at home, the image was clear enough in their body language – Graeme Swann and Kevin Pietersen were not even on the battlefield at the finish, and Stuart Broad was hobbling – and in the post-match words of a plainly bewildered Andrew Strauss. The England captain said they "hadn't counted" on O'Brien hitting such a breathtaking century. The question that ought to have immediately gone back to him was: why not?

"We thought we'd done a reasonable job with the bat," he said, scratching his head, "and [when] we got Ireland five down things were looking pretty comfortable."

Strauss had no clue, either, why they dropped simple catches, gave away overthrows and, with the exception of Swann, bowled naively. But they had played with similarly languid disregard for the disciplines of their sport against the Netherlands, so this was neither an aberration on England's part nor a fluke by Ireland. There were good reasons for the shock and they were firmly based in sensible, powerful batting by Ireland and substandard out-cricket by England.

This was a match that swung on belief in its various shades. Even before a ball was bowled, Ireland believed they had a fighting chance. "You have to believe," their captain, William Porterfield, said. Ireland did. England did – but not from a position of outlandish hope. England's belief was grounded in history and lazy assumptions.

Clearly, there were England players who believed when they had Ireland 111 for five that the rest of the evening was little more than a waste of their valuable dining time. That would now be spent in introspective gloom. And yet, no amount of England-bashing should disguise the magnificence of Ireland's run-chase, which began with James Anderson uprooting Porterfield's off-stump and ended with John Mooney calmly working Anderson's half-volley off his legs to the boundary to win the match with five balls to spare.

There was no luck, Irish or otherwise, in between. Ireland had a couple of transported Aussies in Trent Johnston and Alex Cusack but they were as Irish as a wet mist. Who wasn't? Surely, every true lover of cricket, even if English, was sending good karma Ireland's way as they embarked on their impossible task.

"This is it ... this is it ... This is it!" screamed Sky's David Lloyd as the winning shot rolled into the boundary. "Take a bow Ireland. They can't believe it. I can't believe it. Terrific Ireland." Cusack, impossibly laid back, declared himself "pretty amazed". O'Brien conceded he was "hitting the ball pretty well".

Ireland's biggest challenge was finding a drink in Bengaluru after closing time. England's will be dragging themselves back into some sort of competitive shape against South Africa on Sunday.